Dampers have long been used in mounting systems such as the suspension systems of automobiles and other vehicles. The dampers may and normally do consist of piston and cylinder assemblies whose variable volume chambers are interconnected by an orifice through which hydraulic fluid is displaced and by which it is throttled to an extent that is a function of the size of the orifice. The most commonly used damper assemblies are purely "passive" ones in which the orifice size, and therefore the damping coefficient or function of the damper, are fixed or are subject to only infrequent adjustment. A known deficiency of passive dampers is that the damping forces generated by them will at times amplify, rather than attenuate, the transmission of energy between the members interconnected by the dampers.
"Fully active" dampers or actuator devices, which include high-pressure pumps or other sources of pressurized fluid, are also known. While such devices can achieve excellent attenuation of vibratory and similar forces, their expense, weight and/or bulk make them unsuitable for many applications.
A third type of known damper assembly is the "semiactive" one. A semiactive damper, in keeping with a purely passive damper, has no hydraulic pump, and therefore may be less expensive, heavy and bulky than a fully active damper. However, in contrast to the purely passive dampers, semiactive ones are rapidly switchable between high and low damping states in which the damping coefficient or function thereof is respectively relatively high and relatively low. When switching of a semiactive damper between its states is controlled by and pursuant to a suitable control policy, the force attenuation achieved is much better than that realized by a passive damper and approaches that realized by a fully active damper. Semiactive dampers, and in some instances control policies for them, are disclosed in U.S. Pat. Nos. 3,807,678, 3,995,883, 4,468,050, 4,468,739, 4,491,207, 4,696,489 and 4,742,998; and also in, U.S. Pat. application Ser. Nos. 06/913,067 now U.S. Pat. No. 48L1849, filed 29 Sept. 1986, and 06/945,380 now U.S. Pat. No. 4881A2, filed 22 Dec. 1986, both owned by the assignee of the present application. The disclosures of the aforesaid patents and patent applications are incorporated herein by reference. U.S. Pat. Nos. 3,807,678, 4,491,207 and 4,696,489 each disclose damper control policies based upon the sign (i.e., plus or minus) of the product of the absolute velocity of one of the damper-interconnected members times the relative velocity between such members. U.S. patent application Ser. No. 06/913,067 discloses a semiactive damper control policy based upon the sign of the product of the relative displacement of the interconnected members times the relative velocity between such members. The control procedure disclosed in U.S. patent application Ser. No. 06/945,380 utilizes observers or models to predict the output that would be achieved by a mounting system in each of the different states of its semiactive damper means, and employs a control policy which causes the actual state of the damper means to correspond with the state thereof in the one of the models producing the optimum output estimate.
Although generally producing good results, systems having semiactive damper means controlled in strict accordance with control policies of the foregoing types, or ones similar thereto, may experience shock forces of significant magnitude at some of the times when the damper means is switched from one to the other of its states. The aforesaid shocks may stress system components to such an extent as to shorten their useful life, and/or may cause the generation of objectionable noise. The problem of noise generation appears to be particularly severe in automobile suspensions or other systems containing a resilient deformable member, such as an automobile tire or damper bushing, that is in series with the damper and is capable of storing energy upon deformation and of abruptly releasing the stored energy upon its subsequent return to or toward an undeformed condition.